In the field of optical information processing and optical measurement, the present invention relates to an apparatus which uses coherent light beams to perform optical coordinate conversion and optical correlation processing on two-dimensional images obtained from photographing devices such as CCD cameras.
Among conventional correlators that perform optical pattern recognition, methods using VanderLugt type and Joint Transform type correlators are well known. Either method is based on an optical Fourier transform using lens, so that an input image to be identified, when parallelly moved, can be recognized without a problem (shift invariability). When the input image is rotated or its size changed, the correlation peak intensity described later is not invariable and the recognition capability is found to degrade according to the extent of the rotation and size change.
In conventional apparatuses, when objects--such as characters or device parts whose shapes, sizes, directions and positions differ from one object to another--are to be optically measured and identified, the following steps are generally followed. The input object is first converted into a two-dimensional image (input image); second, when there is any rotation or size change found with the input image, the input image is converted into a desired coordinate system where the correlation peak intensity becomes invariable for such changes as rotation or size changes required for the recognition of the input image; and third, the coordinate-converted image is then measured or identified.
Various coordinate conversions may be used according to specific purposes. For the recognition processing and the rotating angle measurement of objects with differing directions, a polar coordinate conversion is employed. For objects both with differing directions and sizes, the recognition processing, the rotating angle processing and the magnification measurement are performed by means of coordinate conversion from (x, y) coordinates to (lnr, .theta.) coordinates. r and .theta. represent a radius vector and a declination in the polar coordinates.
First, the outline of the optical coordinate conversion method is shown in FIG. 2. With this method, a liquid crystal television 303 displaying an image to be converted and a coordinate conversion optical filter 304 are put close together at the front focal plane of a coordinate conversion lens 307. A parallel-ray coherent beam is radiated from behind the liquid crystal television 303 to form a desired coordinate-converted image on a liquid crystal light valve 308 placed on the rear focal plane of the coordinate conversion lens 307. The coordinate conversion optical filter 304 is made by using a computer generated hologram (CGH). Either of the liquid crystal television 303 or the coordinate conversion optical filter 304 may be placed in front of the other as long as they are put close together.
The liquid crystal light valve 308 commonly uses a TN liquid crystal as the light modulating material. Instead of the liquid crystal light valve 308, a light-addressed spatial light modulator may be used, which employs a BSO crystal (Bi.sub.12 SiO.sub.20) as the light modulating material. The coordinate-converted image is thrown onto the spatial light modulator to display and store the coordinate-converted image, and then the coherent light is irradiated onto the spatial light modulator to read out the coordinate-converted image for such processing as the pattern recognition. Another method is also available, which involves using a photographing device such as a CCD camera instead of the light-addressed spatial light modulator to receive the coordinate-converted image and then entering the image signal into an electrically addressed spatial light modulator such as a liquid crystal television.
Among the known pattern recognition methods using the coordinate conversion is one which uses an ordinary VanderLugt type correlator to perform a preprocessing on the coordinate-converted image. FIG. 3 shows the configuration of the VenderLugt type correlator having the conventional coordinate conversion function. This method is described by referring to the drawing.
As the first step, a matched filter for the reference image is made. At this step, the reference image is displayed on the liquid crystal television 303. The coherent light emitted from the laser 301 is expanded by a beam expander 302 and then split into two beams by a beam splitter 305. One of the split beams passes through the liquid crystal television 303, which is showing the reference image, and the coordinate conversion optical filter 304 and is Fourier-transformed by the coordinate conversion lens 307 before striking the write plane of the liquid crystal light valve 308. In this way, the coordinate-converted intensity distribution image of the reference image is displayed on the liquid crystal light valve 308. The other of the beams split by the beam splitter 305 is reflected by a mirror 306, a beam splitter 309 and a polarizing beam splitter 310 and irradiates the read plane of the liquid crystal light valve 308 to transform the coordinate-converted intensity distribution image into a coherent image. The coherent image passes through the polarizing beam splitter 310 and is Fourier-transformed by a Fourier transform lens 311 before striking a photographic plate 312 as a signal light during the making of the hologram. At the same time, a light beam that has passed through the beam splitter 309 travels through a shutter 313, which is open, and is reflected by a mirror 314 to radiate onto the photographic plate 312 as a reference light during the making of the hologram. At this time, the signal light and the reference light strike the photographic plate 312 at specified angles to form a hologram on the photographic plate 312. The liquid crystal light valve 308 is located at the front focal plane of the Fourier transform lens 311 and the photographic plate 312 at the rear focal plane of the lens. The photographic plate 312 formed with the hologram is taken out and developed and then returned to the original position. The photographic plate 312 recorded and developed with the hologram is called a matched filter.
Next in the second step, the correlation processing is performed. Processing similar to the first step is omitted in the following description or only briefly explained. The second step displays the input image on the liquid crystal television 303. As in the first step, the coordinate-converted intensity distribution image is displayed on the liquid crystal light valve 308, is read out and Fourier-transformed by the Fourier transform lens 311 and then is radiated onto the photographic plate 312, a matched filter. Unlike the first step, the shutter 313 is closed so that the reference light does not strike the photographic plate 312. The light that has passed through the photographic plate 312 or the matched filter is Fourier-transformed again by the Fourier transform lens 315 so that a correlation peak representing the correlation coefficient of the reference image and the input image is produced on a light receiving element 316 located on the conversion plane of the lens. The photographic plate 312 is located on the front focal plane of the Fourier transform lens 315 and the light receiving element 316 on the rear focal plane of the lens.
With the above method, however, there are three problems that arise from the use of the photographic plate as a medium or a matched filter on which the hologram is recorded.
First, it is necessary to form a hologram on the photographic plate and then take the photographic plate out to develop it, requiring time and labor. Second, when the photographic plate, after being developed, is returned to the original position, it is very difficult and troublesome to adjust it exactly to the original position. In particular, the photographic plate must be so set that its light axis is aligned with the center of the hologram and that the plate is not rotated or warped. Third, when the reference image is to be changed, either a new matched filter must be made or the old filter be replaced with a prefabricated matched filter, making it impossible to change the reference image in real time or at high speed. For the reasons mentioned above, it has been impossible to form a pattern recognition apparatus which has a coordinate conversion function capable of a real-time operation in a useful level.
The use of the VanderLugt type correlator poses the following problem. The hologram formed on the photographic plate usually has a large angle of about several tens of degrees between the signal light and the reference light, so that the intervals between interference fringes are very narrow at about several hundred 1 p/mm. Therefore, the process of making the matched filter is easily affected by vibrations and wind. If the interference fringes should oscillate due to such influences, a precise hologram cannot be recorded, deteriorating the pattern recognition capability.